1. Field of the Invention
This invention relates generally to fluid treatment systems for industrial use, and more particularly, to an improved cooling tower water treatment system using redox media.
2. Description of the Related Art
Water is the most commonly used medium for removing heat from industrial equipment. Water has excellent heat transfer capability which is reversible so that the water can be cooled and reused. Typically, water is recycled by the use of a cooling tower which allows a portion of the water to be evaporated. Since water is rarely pure, contaminants in the water are concentrated during the evaporation process. Concentration of the contaminants leads to multiple problems such as scaling, corrosion and fouling by algae, bacteria, and fungi, the treatment of which require the use of chemicals and/or frequent maintenance.
In the past, water was typically treated with chemical conditioners to control scaling, corrosion and biofouling. Chelators and complexers were added to control the formation of scale, inhibitors were added to control corrosion, and biocides were added to control biofouling. In addition to the foregoing, other additives, such as buffers and pH control additives are frequently used. The use of these chemicals adds expense, increases effort to monitor and maintain appropriate chemical levels, and creates disposal problems. Non-chemical systems have been developed, such as magnetic systems and ozone generators, but these have proven to be expensive and, at best, only marginally effective. Ozone systems, for example, have a beneficial effect on the control of biofouling, but have a limited effect on the control of scale formation and corrosion protection.
A unique oxidation/reduction (redox) media has been discovered for treating water by the galvanic reaction that results when the water contacts bimetallic redox media. The bimetallic alloy used in the media for water treatment is preferably a high purity alloy of copper and zinc in an appropriate ratio. The redox media is described more particularly, for example, in U.S. Pat. Nos. 5,510,034; 5,433,856; and related patents. Redox potential (ORP) is a measure of the readiness to part with electrons, and is measured in millivolts (mV). Zinc is more reactive than copper and is more electropositive. In the preferred redox filter media, copper is the permanent cathode and zinc is the sacrificial anode. A single pass through copper-zinc redox filter media rapidly changes the redox potential of water from +200 mV to -500 mV. This change has a dramatic effect on most bacteriologic, solubility, and ionic reactions. The redox media can remove dissolved gases such as chlorine, hydrogen sulfide and methane. It can also remove virtually any soluble heavy metal, help prevent mineral scale accumulation and reduce levels of microorganisms.
More specifically, when cooling tower water is exposed to the redox media, the flow of electrons alters the crystalline structure of the scale-forming compounds. The most common scale-forming compound is calcium carbonate or calcite. When combined with carbon dioxide dissolved from the air, and exposed to heat, calcite is deposited in the heat exchangers, pipes, pumps, reservoirs, and towers used in the cooling system. Left uncontrolled, calcite will continue to grow upon itself until a thick layer of scale is formed. A 0.1" thick deposit of calcite, for example, will reduce the heat transfer ability of a heat exchange by about 40%. The modification of ORP produced by the redox media causes the calcium to precipitate as fine particles of a carbonate compound which is spherical or rod-shaped with rounded edges. Unlike the coarse crystalline calcite scale, the carbonate precipitate cannot grow upon itself and can be removed by filtration.
The medium controls biofouling by two mechanisms. The ORP change produced by contact with the media results in an electrolytic field which most microorganisms cannot survive. Second, hydroxyl radicals and peroxides are formed from some of the water molecules which also adversely impact microorganisms. Finally, the corrosion of metallic surfaces is mitigated by the stabilization of pH to non-corrosive alkaline levels of between 8.0 and 8.5 through the generation of hydroxyl radicals by the redox media. Additionally, the negative impact on bacterial growth prevents the generation of organic acids by the bacteria.
Despite these advantages, the use of the aforementioned redox media has been accompanied by the several disadvantages. In typical prior art systems, the redox media is supplied in a form similar to steel wool. This material is formed around a mandrel. Water flows from the outside of the chamber, through the media, and through the mandrel. In another embodiment, the redox media is supplied in the form of a foam-like product which has been formed into discs. The water flows through a series of these discs for the appropriate contact time. The wool and foam products, being held in a static position, become clogged over time with particulate matter which is precipitated out of the water. Not only does this lead to premature replacement of the media, but over time as clogging occurs, the surface area is reduced and flow is restricted. As a result of this deterioration, performance is reduced and the problems relating to scale, biofouling, and corrosion can recur.
In order to overcome the foregoing problem, a granular media has been used with a downflow pattern in a pressure vessel. While this also results in a buildup of trapped particulates, periodic backwashing allows their removal. This system is an improvement over the wool and foam systems described hereinabove where the media is permanently fouled, but results in diminished performance between backwashing cycles. Another disadvantage of this system is the loss of media due to flow rate of backwashing necessary for particle removal.
It is, therefore, an object of this invention to provide a cooling tower treatment system which does not rely on chemicals for conditioning the cooling water and is inexpensive and simple to maintain.
It is another object of this invention to provide a cooling tower water treatment system which uses redox media efficiently without significant loss of performance between backwashing cycles.
It is also an object of this invention to provide a cooling tower treatment system which maximizes the life of the redox media and system components.
It is still a further object of this invention to provide a cooling tower treatment system which exhibits consistent performance levels over its lifetime.